1. Field of the Invention
The present invention relates to solid state lasers, and specifically to diode-pumped solid state lasers.
2. Description of the Related Art
New generations of diode-pumped solid state lasers utilize stacks of diode laser bars to generate pump beams. The increased power afforded by such arrangements of diode bars as compared to a single diode or single diode bar allows new levels of performance to be achieved in the laser receiving the pump radiation. However, since a stack of diode bars may extend over centimeters or tens of centimeters, additional difficulties are present in efficiently coupling the pump energy from such a diode stack with the gain medium of the pumped laser. These difficulties are in addition to the well known difficulties associated with the highly divergent and non-isotropic output of a single diode laser.
To efficiently couple with another laser cavity, the spatially-extended and highly divergent pump beam from a stack of diode bars may have it""s physical size and angular divergence controlled. In addition, the pump beam""s spatial intensity profile may also be controlled in order to facilitate good matching of the volumes of the activated gain medium and the resonator modes in the laser cavity receiving the pump energy. Such a device may also have heat transfer requirements imposed by the considerable power that only a fractional loss in the coupling of the diode stack output power may entail.
Prior art techniques address the above difficulties in ways that are overly complex with respect to coupling optics, or do not meet the multiplicity of requirements for efficient coupling of extended emitters to laser cavities, or are simply not economical or sufficiently rugged for commercial use. For example, U.S. Pat. Nos. 5,307,430 and 5,323,414 to Beach and Baird, respectively, describe lens ducts. According to the teachings of these patents, the lens ducts rely on Total Internal Reflection (TIR) to guide the pump light from the stack to the gain medium. TIR requires a precisely defined and nearly discontinuous change in the refractive index across the interface between the duct and the duct""s exterior environment. This dependence on TIR makes it very difficult to cool the device since mounting the lens duct to a heat sink or other structure may violate the requirements for TIR. In the prior art, losses in such ducts are typically greater than 20%, resulting in reduced overall efficiencies and potentially problematic thermal management situations. Further, U.S. Pat. No. 5,307,430 and U.S. 5,323,414 do not teach or suggest conditioning of the pump beams intensity profile, mechanical registration or methods of cooling of the coupling device.
U.S. Pat. No. 5,743,901, by Grove, teaches a hollow non-imaging light collection device to collect light from a two-dimensional diode array and deliver it to skin after many internal reflections within the hollow device which act to xe2x80x9cmixxe2x80x9d the light. However, U.S. Pat No. 5,743,901 does not teach or suggest conditioning the output intensity profile or Numerical Aperture (NA) of the device, and not in any manner optimized for pumping a gain medium.
Thus, there is a need in the field for simple, economical, robust, and efficient methods and apparatae for coupling high power, spatially extended emitters to laser cavities. In particular, there is a need for coupling devices that couple the output of high power, spatially extended, diode devices to solid-state lasers for the purpose of pumping the solid state laser. There is also a need for solid state lasers incorporating coupling devices configured to control the pump light intensity profile and numerical aperture (NA) in the gain medium. Such controlled coupler output enhances the performance of the diode-pumped solid-state laser. Further, there is a need for a diode-pumped solid-state laser system with a coupling device that allows for field-replaceability of the high power, spatially extended diode device without requiring readjustment of the laser head.
An object of the present invention is to provide a method and apparatus that couples a highly divergent and spatially extended pump source to a laser resonator.
Another object of the present invention is to provide a method and apparatus that couples a highly divergent output from a stack of diode bars to a laser resonator.
A further object of the present invention is to provide a method and apparatus that conditions the spatial intensity distribution of a pump source.
Yet another object of the present invention is to provide a method and apparatus that conditions the numerical aperture of a pump source.
Another object of the present invention is to provide a method and apparatus that conditions the fluence of a pump source.
Yet another object of the present invention is to provide a laser system with improved ease of replaceability of a pump source for the laser system.
These and other objects of the present invention are achieved in an optically pumped laser. A gain medium is positioned inside of an optical resonator cavity and disposed about a resonator optical axis. An optical pumping source is positioned outside of the optical resonator cavity. A reflective coupler with a coupler body, and an interior volume passing therethrough is positioned proximal to the optical pumping source. Light from the pumping source passes into an entrance aperture of the reflective coupler to an exit aperture of the reflective coupler positioned distal to the optical pumping source. The interior volume of the reflective coupler is bounded by a reflective surface, an entrance aperture and the exit aperture, and is substantially transparent to radiation from the optical pumping source. The reflective surface has a high reflectivity matched to radiation from the optical pumping source. The reflective coupler directs radiation from the optical pumping source into the optical resonator cavity and gain medium, conditioning the numerical aperture and spatial intensity distribution across the exit aperture.
In another embodiment, a pump source, laser resonator and a reflective coupler are provided with the reflective coupler positioned to receive an input from the pump source and deliver an output to the laser resonator. The reflective coupler conditions the angular divergence and spatial intensity profile of the output of the pump source to render the output of the laser resonator nearly constant as the pump source is replaced.
In another embodiment of the present invention, a method of optically pumping a gain medium includes conditioning a fluence of an optical pump source beam with a reflective coupler prior to illuminating a gain medium with the optical pump source . An angular divergence or numerical aperture of an optical pump source beam is conditioned with a reflective coupler prior to illuminating a gain medium with the optical pump source beam. Additionally, a spatial intensity distribution of an optical pump source beam with a reflective coupler is conditioned prior to illuminating a gain medium with the optical pump source beam.